Conventional modulators used in pulsed accelerators produce electrical pulses that drive radio frequency (RF) tubes. The RF output from these tubes produces an electric field in acceleration cavities and the electric field accelerates the charged particles. The acceleration cavities can either be normally-conducting or superconducting. The pulses in normally-conducting cavities are typically only a few microseconds long to minimize the resistive dissipation in the cavities. Since there is very little dissipation in superconducting cavities, the pulses for these are much longer, typically of order a few milliseconds, because this reduces the peak power required.
One example of a current long-pulse modulator is at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. The specification for this modulator is shown in Table 1 below:
TABLE 1SNS modulator specification.Voltage85 kVCurrent165 APulse Width1.5 msRepetition Rate60 HzVoltage Flatness1%
Ideally, such a pulse modulator would provide very flat pulses (constant voltage), at high efficiency and low cost, in a compact and reliable configuration. These factors present a number of challenges. One challenge associated with long, e.g., millisecond scale, pulses at high current is that they couple substantial charge, which decreases the voltage on the capacitors in the modulator. This voltage decrease is often referred to as droop. It is typically desirable to limit the capacitor droop to a few percent or less. While it is conceptually possible to increase the capacitance to limit this droop, the size of a capacitor needed to achieve this is typically too large to be practical. For example, limiting the capacitor droop to 1% in the SNS modulator discussed above would require a capacitor bank that stores about 1.1 MJ. Instead, an alternate means of reducing capacitor droop regulation is needed.
Another problem associated with pulsed modulators is that they do not draw constant power. Pulse modulators typically draw power in a pulsed fashion, which transiently decreases the voltage on the AC lines, a phenomenon known as flicker (named because it makes electric lights flicker). Flicker can be highly disruptive to both local power users and the grid itself.
There are several conventional modulator designs which have been deployed in large accelerator systems. Each of these has one or more drawbacks in its implementation, as discussed below. One of the significant challenges to these designs is the need to regulate voltage droop in a long-pulse modulator.
Historically, droop regulation has been done by dissipating power with a vacuum tube. However, the power dissipated in a vacuum tube would be substantial. For example, a system with 10% droop would require at least 5% average dissipation, which would be 63 kW in each of the fifteen SNS modulators.
In high-power modulators, it is desirable to regulate droop without such dissipation. There are several techniques currently used to regulate droop without dissipation. One is a power converter modulator, such as the one currently implemented at the SNS. This power converter design includes a semiconductor switch bridge circuit that produces pulses of alternate voltage. The pulses are stepped up by a transformer, then rectified to deliver the output pulse. To provide a flat output pulse as the voltage on the bus capacitor droops, the width of the alternating pulses is increased over time, similar to pulse-width modulation in a switching power supply.
One problem with this implementation is the bridge circuit switches full power repeatedly during a pulse. Such a design requires large switching transistors. To reduce the switching losses in the transistors, the bridge circuit may be resonant. However, this adds significant complexity to the device. Developing the modulator system for the SNS has taken ten years and has had multiple failures in the switching transistors, the resonant capacitors across the IGBTs, and the high-voltage transformers. Moreover, the droop regulation is not yet functional.
Another conventional system used to regulate droop in a long-pulse modulator is a bouncer modulator. The bouncer modulator compensates for the droop with an auxiliary capacitor and inductor. Both the main power supply and the bouncer power supply for the bouncer modulator need to be well-regulated to provide an accurate output voltage, and therefore need to be switching power supplies. A high-power main switching supply is significantly more expensive than an SCR-controlled supply, which cannot be used in this architecture.
Another conventional design is a pulse-step modulator, which operates with multiple stages that are charged in parallel by a transformer then switched in series. The power flows through a boost regulator, which is controlled to provide a constant power draw which reduces flicker. One problem with the conventional pulse-step modulator is that the boost regulators need to be large enough to carry all the power. This is much more expensive than an SCR-controlled power supply. The pulse step modulator is also large because of the high-voltage charging transformer.
Yet another conventional system used to regulate droop in a long-pulse modulator is a Marx generator. The Marx generator is similar to the pulse-step modulator because multiple stages are charged in parallel, then switched in series. The main difference is the Marx generator charge current passes up the chain of stages, while the pulse-step modulator charges through a transformer with multiple secondary windings. Additional Marx stages can be switched on during the pulse to compensate for droop, which add to the overall cost and complexity of the modulator. Such a design also requires an expensive switching power supply to eliminate flicker.
Accelerators can also be designed to operate continuously rather than being pulsed. While voltage regulation is important for these systems as well, the major concern is ripple on the output pulse rather than droop. An example of a continuous accelerator is the Advanced Photon Source (APS) at Argonne National Laboratory. The APS modulator parameters are shown in Table 2 below. The large amount of ripple is because the high-voltage power supply is SCR-controlled. While a switching supply would produce much less ripple, it would be more expensive.
TABLE 2APS modulator parameters.Voltage100 kVCurrent 20 APresent Ripple 1.3 kV p-p
There are several conventional approaches to mitigate the ripple from the power supply. In one example, a modulating-anode supply reduces the ripple voltage between the cathode and the modulating anode, and low-level RF feedback compensates for the remaining ripple. Although this method is somewhat effective, it may be necessary in some instances to reduce the ripple even further.
Thus, there is a need for a system which regulates the output of a high-voltage, high-power, DC supply to reduce capacitor droop and DC ripple.